Pressure gauges enjoy very extensive commercial and industrial use and are consequently regarded as high production items. Because of such wide use, they are supplied by a plurality of manufacturers and sold in very price conscious competition. Each manufacturer instinctively strives to reduce product costs by improvements, however marginal, which reduce labor and/or materials that can contribute to cost savings in the end product.
Commonly affording pressure sensitivity in the pressure gauge is the Bourdon tube being a tube of a pressure-tight construction having a free end displaceably movable in a well known and predictable manner in response to pressure changes supplied at its inlet. To translate tube movement into values of pressure, a pointer opposite a calibrated dial plate is displaceably driven by the free end of the tube.
It can be appreciated that the Bourdon tube under in-service conditions encounters operational stresses which in effect contribute to its characteristic displacement in response to pressure changes to which it is exposed. At the same time, such stresses if not properly accommodated can produce premature failure of the tube. To minimize possibility of the latter while retaining its characteristic displacement property, it has long been the practice to produce Bourdon tubes from a homogeneous product formed either from seamless tubing or from welded and drawn tubing. In the seamless tubing variety, tubing is produced by first piercing a round billet and thereafter reducing its size in a plurality of sequentially effected cold drawing operations with intermediate anneals. Welded and drawn tubing, by contrast, is produced by fusion welding strip to produce a round master tube. The tube is then reduced in a plurality of repeated cold drawing and annealing steps until finally achieving the required final Bourdon tube equivalent round.
The purpose of cold drawing and annealing in accordance with these prior practices has been not only to achieve final size but to homogenize the weldstructure, since it is well known that the mechanical properties of weldments are not equivalent to that of wrought alloy forms. Consequently, by drawing and annealing a cast weld microstructure, welded tubing is gradually changed to more nearly correspond with that of wrought strip material.
It can be appreciated therefore that producing a Bourdon tube from tubing formed from either of the mentioned prior art approaches requires extensive metallurgical mill processing in order to achieve the desired end result. Such processing however is known to contribute significantly toward the end cost thereof. Despite recognition of the problem, it has not been known heretofore how to produce such tubing in a less costly manner.